The present invention relates to a method of enlarging the width of a continuously cast strand. More particularly, it relates to a method of restarting a continuous strand casting process for producing a cast strand having a wider width than that of a previously cast strand wherein the top end of the newly cast strand is connected with the tail end of the previously cast strand while the tail end of the previously cast strand is still in the mold.
In prior art processes when the width of a cast strand was to be enlarged in a continuous strand casting process, the casting was stopped; the cast strand was removed from the mold; the mold was set so that a product of the desired wider width could be produced; a starter bar sometimes referred to as a dummy bar was inserted into the bottom opening of the mold; the opening between the mold wall and starter bar was sealed with asbestos; and thereafter the casting process was restarted. Such a procedure was apparently timeconsuming.
Recently various methods and molds have been proposed for enlarging the strand width in a continuous casting process.
Japanese patent application No. 51-55278 (Japanese Patent Laid-open Specification No. 52-138437, published on Nov. 18, 1977) discloses a method of enlarging the width of a continuously cast strand wherein the width of the strand being cast is enlarged by gradually outwardly moving narrow mold pieces 51, which define the width of the cast strand, while continuing the casting process (See FIG. 1). However, this method involves various difficult problems. The rate at which the narrow mold pieces are moved is preferably controlled in accordance with the casting rate, and thus, an additional mechanism for providing such a control was needed. The narrow mold pieces must be moved while keeping the smallest possible openings between walls of the widened mold pieces and the abutting ends of the narrow mold pieces. Consequently, motors of a large capacity were required for driving the movement of narrow mold pieces. By this method it is impossible to significantly enlarge the width of the cast strand quickly. The product strand has a substantial lengthwise portion, in which the width gradually varies and which cannot be further processed in the same manner as conventional strands, leading to a reduction in the yield of usable strand. Furthermore, failure to properly control the rate, at which the narrow mold pieces are moved, with respect to the casting rate as well as any leakage of molten metal through any openings between the abutting faces of the wide and narrow mold pieces, will pose a problem of "break-out" accidents.
According to a method disclosed in Japanese patent application No. 50-65654 (Japanese Patent Laid-open Specification No. 51-141721, published on Dec. 6, 1976), inwardly and outwardly movable rest plates 52 are provided beneath the bottom ends of narrow mold pieces 51 (See FIG. 2). When the width of the strand being cast should be enlarged, the casting is stopped; the rest plates 52 are inwardly moved until the inner ends of the rest plates 52 come in contact with the strand (FIG. 2a); the narrow mold pieces are outwardly moved until they define the desired enlarged width; the openings so formed between the strand and the walls of the mold pieces are filled with a metallic cooling materials 53; a molten metal is poured into the mold (FIG. 2b); the process is restarted; the rest plates 52 are outwardly moved away; and the casting process is restarted for producing a wider strand. In this method, however, means for inwardly and outwardly moving the rest plates are needed, rendering the installation more complicated. Furthermore, a serious disadvantage is involved in that because of a great difference of the solidified shape between the middle and edge portions of the width of the strand, a "break-out" accident is liable to occur upon drawing the strand from the mold.
Japanese Utility Model Registration Application No. 47-71348 (Japanese Utility Model Laid-open Specification No. 52-16255, published Apr. 12, 1977) discloses a method of enlarging the width of a continuously cast strand using a mold wherein each of the narrow mold pieces is divided into upper and lower pieces 51a and 51b as shown in FIG. 3. In practice when the tail end of the cast strand has reached the level of the lower ends of the upper narrow mold pieces 51a, oscillation of the mold and drawing of the strand from the mold are stopped, and the upper narrow mold pieces 51a are outwardly moved to define the desired enlarged width of the strand to be cast therebetween. On the shoulders so formed on the upper ends of the lower narrow mold pieces 51b within the mold cavity, cooling boxes 53 filled with a cooling material are placed (FIG. 3a). A molten metal is poured into the mold. After the skin of strand, especially that portion contacting with the cooling boxes, has been sufficiently solidified, the lower narrow mold pieces 51b are outwardly moved until they define the new width of the strand. The casting process is then restarted (FIG. 3b). This method, however, suffers from such disadvantages that the division of each narrow mold piece into two parts renders the structure of mold more complicated; care should be taken so that no gaps will be formed between the upper and lower narrow mold pieces in the course of normal casting; and the extent of enlarging the width is limited by the thickness of the upper and lower narrow mold pieces. Furthermore, any leakage of molten metal through any openings between the abutting ends of the upper and lower narrow mold pieces at the time one of the upper and lower pieces is slidably moved might invite a "break-out" accident.